1. The Field of the Invention
The instant invention relates to solid oxide fuel cells and particularly to ceramic interconnect materials having good electrical properties.
2. The Prior Art
Solid oxide fuel cells (SOFC'S) are structured to convert the energy of combustion directly to electrical energy. Low molecular weight, residue-free gases, especially natural gas, carbon monoxide, hydrogen and other clean-burning gases, are employed as fuels. A solid electrolyte, e.g. ZrO.sub.2, which rapidly transports oxygen ions is an essential component in SOFC's.
Typical SOFC's are illustrated in the following U.S. Pat. Nos.:
4,476,198 Ackerman, et al.
4,816,036 Kotchick
4,476,196 Poeppel, et al.
The fuel cell operation is shown schematically in FIG. 4, wherein oxygen is introduced at the cathode, dissociates to form oxygen ions by picking up electrons from the external circuit. The oxygen ions flow through the electrolyte (which is at an elevated temperature .about. 700.degree. C. or more) to combine with hydrogen, for example, in a combustion reaction (exothermic). The electrochemical heat of reaction and the internal resistance maintains the fuel cell at an efficient operating temperature, i.e., one at which the ceramic electrolyte, typically ZrO.sub.2, is an efficient conductor of oxygen ions. The combustion reaction (half cell reaction at the anode) is as follows: EQU O.sup.= +H.sub.2 .fwdarw.H.sub.2 O+2e
The electrons freed by this reaction are available as electrical energy to perform useful work. The circuit must be complete so that the electrons are available at the cathode-electrolyte interface to participate in the dissociation of oxygen molecules into oxygen ions, as follows: EQU O.sub.2 +4e.fwdarw.20.sup.=
Ceramic interconnect devices interconnect one cell to another electrically and act as channels for both the gaseous fuel and oxygen, as illustrated in FIG. 5. While FIG. 5 shows only two cells connected by a single interconnect, it is typical that a plurality of interconnects are used to form a "stack" of cells, thus serially connecting one cell to another from an electrical standpoint.
The interconnect must be a good conductor of electricity, have a coefficient of thermal expansion (CTE) which closely matches the electrolyte, e.g. zirconia, and be thermodynamically stable simultaneously at high oxygen partial pressures in oxygen or air and low oxygen partial pressures in the fuel gas at cell operating temperatures. Many materials may satisfy one or two of these requirements, but the lack of effective, long lasting interconnects has thus far retarded the development of a commercially usable fuel cell, such as those made of lanthanum strontium chromite (LSC).
The functional requirements of the interconnect in solid oxide fuel cells currently limit the choice of materials to lanthanum chromite based perovskite compositions. Lanthanum chromite exhibits a close thermal expansion match to the zirconia electrolyte and high stability of the wide range of oxygen partial pressures in SOFCs, 0.2 atm on the air side to 10.sup.-18 -10.sup.-22 atm on the fuel side, at cell operating temperatures of 800-1000.degree. C. The required high electronic conductivity can be achieved by appropriately doping the perovskite material with aliovalent dopants. "Aliovalent" for the purposes of the present invention means "of different valence" (or electronic charge) compared to the base elements. For example, in LaCrO.sub.3, where La and Cr both nominally have a +3 charge, aliovalent dopants might include Ba, Sr, Ca, Mg, Zn, etc., which have a +2 charge. Alkaline earth dopants such as Sr and/or Ca are typically used to obtain the high conductivity exhibited by these compositions.
These compositions, however, show a loss of lattice oxygen in the fuel atmosphere. Ideally, an interconnect should conduct electrons alone, and not oxygen ions. The formation of oxygen vacancies in such compositions results in diffusion of oxygen ions due to the large chemical potential gradient across the interconnect. This oxygen diffusivity manifests itself as an oxygen leakage current (the amount of oxygen ions per unit area per unit time that permeates through the interconnect material by oxygen ion conduction -- this phenomenon may also be called "ionic leakage current"). The mechanism is similar to the transport of oxygen through an electrolyte. This leakage current or ionic conduction is a parasitic loss to the system since no useful energy can be harnessed by the mechanism. The leakage current has, in turn, several effects. First, the cell efficiency is lowered by non-current delivering oxidation of fuel. This is an acute problem in planar SOFCs, in which thinner "web" cross-sections between the cross flow ribs are employed to minimize the ohmic loss in the interconnect. In contrast, tubular design geometries, such as employed by Westinghouse Electric Corporation, facilitate a much smaller exposed area for possible oxygen leakage currents. A second consequence of an oxygen leakage current is a reduction of the Nernst driving force for the current producing electrochemical reaction across the electrolyte, owing to consumption of the fuel and a higher water content. Thus, it is of great interest to understand and minimize the magnitude of oxygen diffusion in such interconnect materials.
In addition to the electrochemical performance penalties, the oxygen leakage behavior also manifests as a structural problem in planar SOFCs. The loss of lattice oxygen results in an expansion of the lattice structure and macroscopic expansion of the sintered ceramic body. Thus in an operating fuel cell the dimension of the interconnect on the fuel side becomes larger than the dimension on the air side resulting in what is termed as `fuel induced warpage`. The magnitude of such warpage is strongly affected by the lateral dimensions of the fuel cell. For example, finite element analysis indicates that for a chemically induced lattice expansion on the fuel side of 0.2%, an interconnect of 10.times.10 cm dimension will have a warpage of 0.18 cm at the edge at 1000.degree. C. when exposed to fuel gas on one side and air on the other. This chemical expansion value is typical for a 16% Sr doped lanthanum chromite. The finite element model of a graded chemical expansion through the thickness of the interconnect is shown pictorially in FIG. 6.
Thus conventional alkali earth doped chromites do not function well in SOFC applications. Magnesium doped chromite materials have been disclosed by Westinghouse but suffer from densification problems, requiring either controlled atmosphere sintering or high temperatures. Thus, there is a need to develop an alternative composition of chromite which facilitates ease of sintering, exhibits minimal warpage (i.e., chemical coefficient of expansion), stability and high enough conductivity at operating temperatures.